1. Field of the Invention
This invention relates to a ballast for at least one gas discharge lamp, and to a method of operating a gas discharge lamp having such a ballast.
2. Description of the Related Art
Today, with high quality ballasts for gas discharge lamps, the lamp electrodes are usually preheated before the ignition voltage is applied therebetween. It has been found that through this measure the lamp life is extended to a considerable degree.
As is described for example in EP 0 594 880 A1, gas discharge lamps are as a rule operated on a series oscillation circuit, whereby an oscillation circuit capacitor lies, as a rule in parallel to the discharge path of the gas discharge lamp. The electrodes of the lamp are formed as heating coils through which the current of the oscillation circuit flows when the lamp is not ignited. In a preheating operation, the frequency is so varied with respect to the resonance frequency of the resonant circuit that the voltage applied across the resonance capacitor and thus across the gas discharge lamp does not cause ignition of the gas discharge lamp. In this way, there flows a substantially constant current through the lamp electrodes configured as coils, so that these are preheated. After conclusion of the preheating phase, the frequency is set in the region of the resonance frequency of the resonant circuit, by means of which the voltage across the resonance capacitor so increases that the gas discharge lamp ignites.
It is usual today, with high quality ballasts, to provide also for dimming operation of the gas discharge lamp. It has now been found that with strong dimming a premature aging of the gas discharge lamp occurs. For this reason it is necessary to provide an arrangement with which the electrodes of the gas discharge lamp can be heated also in ignited operation. In particular it is advantageous to set the heating of the electrodes in dependence upon the degree of dimming, i.e. the more strongly the lamp is dimmed that is, the darker it is--the more strongly must the electrodes be heated.
A suitable circuitry arrangement for this purpose is described in EP 0 589 081 A1. This has the primary winding of a heating transformer in the resonant circuit, the secondary windings of which are connected in parallel to the terminals of the heating coils. In this way it is possible also in ignited operation to supply energy to the heating coils. Further, there is provided a controllable switch in parallel to the primary winding of the heating transformer, which switch bridges the primary winding when needed and thus simultaneously prevents the heating of the heating coils.
This arrangement has, however, the disadvantage that through the provision of the primary winding of the heating transformer in the series resonant circuit this damps the resonant circuit at least so long as heating of the coils is necessary, i.e. the switch connected parallel to the primary winding is open. This leads to a detuning of the resonant circuit, so that reliable ignition and thus dependable operation can no longer be unrestrictedly ensured.